Journalists should learn some physics:
Astronomers announced last week that they had found what might be
the first habitable planet outside the solar system. Known poetically
as Gliese 581c, the new planet is only five times as massive as the Earth
and inhabits a sweet zone around a dim red star in Libra where it is
neither too hot nor too cold for liquid water.
The point the reporter eventually makes is that, at 20 light years away, it would take about 300,000 years to get there at Voyager velocity. Perfectly true. However, there are still two problems here. The first is that the force of Earth-gravity on Earth-bound objects is directly proportional to the gravitational constant here on the planet's surface. That number (we call it "g") is itself directly proportional to Earth's mass, but inversely proportional to the square of Earth's radius. The fact that Gliese is five times as massive as Earth would might well pose a problem. Now it turns out that planets (and especially rocky Earth-like planets) tend to have comparable gravitational constants, and so their surface gravities aren't wildly different from each other. But humans are evolved to live at earth density. Not at moon density (about 20 percent of Earth's gravity) and not at Jupiter gravity (about 200 percent of Earth's gravity). It also happens to be the case that a rocky planet's have similar densities and so we can be pretty sure that Gliese's gravity is greater than Earth's gravity by a noticeable margin--my guess is by a factor of two. That means humans on the Glesian surface would experience--I would guess--a risk of bone fractures, muscle strains, and circulation issues, at least until we pumped up a bit. I don't really know what the heart can handle.
The second problem is that we find these planets by looking at Doppler perturbations in their host star's rotation around its own axis. We infer their existence. We don't know what their atmospheres look like, and therefore we can't possibly say what their surface temperatures are. There could be a tremendous greenhouse problem (a la Venus whose atmosphere is a smoldering blanket of toxic greenhouse nastiness) or a tremendous radiation problem (a la the moon whose lack of an atmosphere leaves it wide open for some of the higher-energy photons that the sun emits). We just don't know.
One possibly helpful fact, though, is that Gliese's sun is a dim red star. "Weaker" stars have longer lives than "stronger" stars and that means that Gliese's sun will likely long outlast our own sun, which will turn into a red giant and cook--and in some cases completely envelop--the planets in this solar system in a few billion years. The downside, though, is that our eyes (and our plant life and a host of other things) are evolved to be bombarded by our sun's light, which has a spectral peak at a green wavelengths. (That's why green laser pointers are better for using in lecture halls, though red laser pointers are cheaper and therefore hegemonize the laser pointer market). Living on a planet with a red sun would be odd. Who knows? Organic plantlife there might even, in some seasons, become invisible to our eyes.
